Asynchronous machines are today used in a wide variety of applications. Asynchronous machines have a large number of advantages. It has generally a simple construction and is typically very safe in operation, which implies a long life. However, the starting and stopping of asynchronous machines requires additional considerations. Applying full voltage at once will result in very large current peaks, which results in a voltage drop. For large motors, in particular in combination with weak power networks, such voltage drops may cause large disturbances in the network. A slow start of the machine is therefore often requested, at least for larger motors. In large and complicated applications, the asynchronous machines are sometimes equipped with speed regulators, sensing the actual slip and talking measures to obtain the required operational conditions. However, for simple applications, solutions based on actual slip sensing are generally too expensive. A common way to solve the starting and stopping problems is to add a device—a soft starter/stopper—which ramps the voltage supplied to the asynchronous motor up and down according to predetermined time schedules.
FIG. 1a illustrates typical torque curves as a function of the rotational speed for an asynchronous motor used for driving a pump. The broken line corresponds to the motor and the full line corresponds to the load. At normal operation, the operational point is situated at the intersection of the curves, i.e. at the point A. Upon stopping the motor, the most common prior art soft starter/stopper makes use of a linear ramp-down of the voltage. The torque available from the motor will during such a procedure be scaled down in the same rate. FIG. 1b illustrates a situation, where the motor voltage has been reduced to give a new operational point B. This operational point B is, however, unstable, and a small disturbance may lead to a small decrease in torque, which in turn leads to a sudden change of the operational point to point C instead, where the torque of the machine is significantly reduced.
In systems where the load of the motor has a high momentum of inertia, this does not result in any problems. The rotational speed of the motor anyway will decrease slowly, because of the inertia of the load, until the motor stops. However, in many applications, such as for centrifugal pumps, the inertia of the load is low, and a sudden change of operational point of the motor will lead to a drastical change in motor speed. In the example in FIGS. 1a and 1b, the rotational speed will decrease rapidly by about 50%. This results in typically in large transients in the system to which the pump is connected. In a system where e.g. fluids of a relative high density are transported, a significant pressure wave can be created, which may damage the pump equipment as well as other parts of the system. This can include damaging vibrations or hammering.
Another problem area is the application of asynchronous motors to stirrers. Many fluids have a speed dependent viscosity. This means that a stationary fluid may have a considerably higher viscosity than the same fluid in motion. At the point when the fluid starts to move, the viscosity is significantly reduced. The load curve of a stirrer in such a system may e.g. look like the curve in FIG. 2. The curve shows a rather high initial load at zero speed, indicated by the filled circle, but the load decreases abruptly when the speed exceeds zero, as indicated by the dotted line. Upon starting a pump in such a system with prior art soft starters, the voltage is ramped-up until the torque of the stationary load is reached. The motor will not move until this point is reached. When the motion eventually starts, the load torque is significantly reduced, and the motor speed is suddenly increased. Such sudden change of rotational speed may damage the system due to vibration and/or mechanical damage.
In the U.S. Pat. No. 5,008,608 of Unsworth et. al. a controller for starting and stopping electric motors is disclosed. The controller comprises thyristor control means on each phase. The controller measures the interval between the voltage and the current when one phase of the motor makes a zero crossing. Based on this value and a reference value, the trigger times of the thyristors are controlled. To start and stop the motor, the reference value is gradually increased or decreased. The device prohibits sudden changes in torque upon stopping the motor. This solution is based on zero-crossing detection of both voltage and current. Such devices are, however, too expensive to be included in simple applications.
In the patent EP 0 621 680 B1, method for limiting the torque in soft starting of a three-phase asynchronous motor is disclosed. The regulation is made by thyristor control based on measurements of the active power of the machine. The torque is controlled, by regulating on the active power. To measure the active power, the current, voltage as well as the relative phase angle have to be measured. Also this solution is based on expensive components, unsuitable for many applications. Furthermore, this disclosure does not mention any problems in stopping the machine.